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Spiroaminals – Crystal Structure and Computational Investigation of Conformational Preferences and Tautomerization Reactions
Author(s) -
Loerbroks Claudia,
Böker Birte,
Cordes Jens,
Barrett Anthony G. M.,
Thiel Walter
Publication year - 2014
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201402576
Subject(s) - tautomer , chemistry , aminal , steric effects , anomeric effect , imine , protonation , amine gas treating , computational chemistry , stereochemistry , crystallography , anomer , organic chemistry , catalysis , ion
We report the first X‐ray structure of a spiroaminal hydrochloride. The chiral spiroaminal crystallizes as a racemic hydrochloride in the monoclinic space group P 2 1 / n and adopts the thermodynamically most stable conformation. Density functional calculations on several spiroaminals were used to establish correlations between trends in conformational energies, steric repulsions, and anomeric effects and to reveal the mechanism of the ring‐opening tautomerization reaction. In the unsubstituted and backbone‐substituted spiroaminals, the aminal tautomer is thermodynamically preferred. N ‐Substituted spiroaminals favor the amine/imine form for steric reasons, except for those with bridging N,N′ groups. The tautomerization from the aminal to the amine/imine is endergonic and kinetically hindered in the neutral species but quite facile after protonation. Anomeric effects lower the barriers but are less important than steric factors for relative energies.